In scanning laser radar (LADAR) ranging systems, narrow laser pulses are generated and transmitted. Each transmitted pulse strikes a target of interest and reflects a portion of the transmitted pulse back to a receiver associated with the transmitter. Such LADAR systems are commonly installed in projectiles such as missiles to determine target type, location, and the range to a target. The time duration between the transmission of a pulse and the reception of the return pulse is convertible into a range.
In known systems, the transmission of the outgoing pulse starts a ramp function within the electronics of the LADAR system and the capture of the return signal terminates the ramp function. The height of the resulting ramp is directly proportional to the range to the target.
However, the shape of the return signal is not a sharp pulse. It has contour and shape. Furthermore, the shape of the pulse itself contains a great deal of useful information. For example, the shape of the return pulse may provide information regarding the size and configuration of the target. The shape may help the LADAR distinguish between different types of targets. And, the analog method of laser ranging inherently includes uncertainties in when to stop the ramp function due to variations in threshold levels.
Furthermore, ideally such an analogue system terminates the ramp functions at the center of a pulse since a pulse has a finite duration. But since the height of a pulse cannot be predetermined, a threshold must be set and this results in inaccuracies of many feet in range.
Other known systems start a fast counter when a laser pulse is transmitted and terminate the counter when the return pulse is detected. The value of the counter is then read out and the value is proportional to range. This method suffers from the same threshold uncertainties as the analogue ramp methods.
Thus, there remains a need for a digital system that reduces the uncertainties in determining a range to a target. Such a system advantageously includes a method of digitizing a return pulse and analyzing the pulse for characteristics of the pulse other than range.